The heating, ventilating and air conditioning of various building spaces typically requires that air flows from one area to another. The required air flow is usually accomplished by forcing or otherwise allowing air to pass from one area to another through duct work or apertures located in walls, floors, roofs or ceilings. While such air flows are necessary to maintain the desired heating, ventilating and air-conditioning environment under normal conditions, air flow through these openings is undesirable during a fire because air flow from one area to another can contribute to the spread or intensity of the fire. For this reason, ducts and other air flow apertures frequently are fitted with fire dampers which can block the flow of air through these apertures when the dampers reach a predetermined temperature.
Fire dampers used in the above-described manner generally contain a plurality of spring-loaded metal blades or louvers held in an open or retracted position within a damper frame by a thermally fused link. When the temperature of the link reaches the melting point of the link material, the link melts or otherwise deforms. Without the louver retention provided by the link, the spring-loaded louvers move into a closed position across the fire damper air flow area, thereby preventing the spread of fire or heated air through the duct or orifice.
Fire damper assemblies used in the above-described manner must be designed to survive severe thermal and mechanical conditions. Because damper blades and frames must withstand the high temperatures encountered in a fire, fire damper components are usually constructed from a metal such as steel. These metal damper components tend to expand as their temperature increases. Therefore, fire dampers must be designed so that they will be able to function when their components are in the expanded configuration expected during fire conditions. Additionally, damper assemblies used in critical applications such as in nuclear power plants must be designed to survive and function during or after seismic events of a magnitude specified in government regulations or in other building or fire codes.
One method of dealing with the thermal expansion of fire damper assemblies is to mount the assembly so that an expansion gap remains between the damper frame and the damper mounting aperture on at least one horizontal and one vertical side of the frame. When the high temperatures associated with a fire are encountered, the assembly frame expands into the gap. The frame expansion provides additional clearance for the simultaneously expanding damper louvers, which might bind or otherwise fail to move to a closed position if the expanded blades were forced to move through an nonexpanded frame.
Expanding frame dampers such as those described above present several design difficulties. Primarily, these difficulties stem from the need to mechanically secure the damper within the mounting aperture while still allowing the damper frame to expand. Additionally, most applications require that any expansion gap left between the damper frame and the mounting aperture remain substantially airtight under both normal and fire conditions.
Various prior art designs have compromised either the mechanical mounting of the damper frame or the free expansion of the damper frame. For example, U.S. Pat. No. 4,579,047 to Zielinski discloses a damper having an inner and an outer frame with an expansion gap between the two frames. To accommodate expansion of the damper frame, Zielinski incorporates resilient spacers within an expansion gap. These spacers provide sufficient compressive force on the inner frame to maintain its position within the outer frame while at the same time purportedly yielding to an expanding damper frame under fire conditions. In this design, the free expansion of the frame appears to be compromised because the expanding frame must overcome the compressive force of the resilient spacers as the frame expands into the expansion gap under fire conditions. Additionally, Zielinski's need to accommodate frame expansion in a direction perpendicular to the plane defined by the damper opening requires that his damper frame be maintained between angle iron spacers mounted further apart than the nonexpanded depth of his damper frame. The spacing of the angle iron spacers appears to present a potential for slidable movement that could comprise the physical stability of the damper assembly under vibratory conditions such as those encountered in a seismic event.
Accordingly, a need exists for a fire damper assembly which permits substantially unrestricted thermal expansion of the fire damper frame under fire conditions while at the same time providing for a mechanically stable mounting of the damper frame within the mounting aperture under non-fire conditions.